IN GYMNOSPERMAE 153 



with two pairs, and had its vegetation been continued under the unfavourable 

 conditions it would have finally produced the primary leaf-form. In other experi- 

 ments plants which had developed more than four pairs of pinnules went back 

 to the stage shown in leaves 6 and 7 in Fig. 93. In old plants possessing 

 stronger shoot-axes and abundant reserve-material, such a reduction is usually 

 impossible because at the vegetative point primordia of leaves already exist the 

 differentiation of which is determined, and because the plant has sufficient plastic 

 material available to make it at first less liable to the influence of external un- 

 favourable conditions. 



4. GYMNOSPERMAE 1 . 



The seedlings of the Cycadeae furnish nothing special in their configuration 

 for our consideration 2 , but those of many Coniferae on the other hand have 

 attracted much attention as many of them play an important part in horticulture. 

 The juvenile form, especially in many Cupressineae, can be ' fixed,' that is to say, 

 we can artificially prolong or make theoretically unlimited what is normally only 

 a developmental stage more or less rapidly passed over. This can be effected in 

 two ways n either lateral shoots of the seedling which show the characteristic 

 juvenile form are used as cuttings, or the chief shoot is removed above the basal 

 lateral shoots and these then grow stronger and retain the juvenile configuration, 

 whilst in normal circumstances they would have been suppressed by the stronger 

 and differently formed chief shoot. Even at a later age the plant retains by 

 preference at the basal region the capacity to produce shoots of a juvenile form. 

 A few examples may be given here : 



i. Pinus. The pines produce as is known only brown 'scale-leaves' upon 

 the shoot-axes which form the framework of the plant, and these scales act as 

 bud-scales and fall off soon after the unfolding of the bud. Spur-shoots arise in 

 their axils which bear two or many needle-like leaves ; in Pinus monophylla there 

 is only one. The seedlings however produce foliage-leaves upon the long shoots 

 following the cotyledons. In Pinus Pinea this continues for many years 4 , but 

 in Pinus sylvestris the formation of primary leaves disappears in the second year, 

 they are still formed at the base of the elongating shoot but scales take their 

 place in its upper part, and in their axils the spur-shoots arise. It is stated 

 in the literature that the juvenile form can be ' fixed ' by cuttings, but I have 

 not been able to satisfy myself upon this point as the cuttings did not grow in 

 my experiments. The needle-like primary leaves have, according to Kaufholz 5 , 

 a simpler anatomical structure than the subsequent foliage-leaves ; the provision 



1 See the papers by Carriere and Beissner quoted by me in Flora, 1889. 



2 The primary leaves of Ginkgo are arrested formations. 



3 The first method has been long known, the second was first brought into notice, so far as 

 I know, by myself. It has special interest because it illustrates the influence of relationships 

 of correlation. Normally the shoots showing the juvenile form are suppressed by those of the adult 

 form ; if the latter be removed the former grow more strongly and acquire a duration much 

 longer than normal. 



4 The duration and degree of development of the juvenile form varies in the same species. 



5 Kaufholz, Beitrage zur Morphologic der Keimpflanzen. Inaug. Dissertation. Rostock, 1888. 



